The present invention relates to a method of calibrating a heater system.
Ice protection systems are one example of a heater system for protecting against the build-up of ice on a structure. One common application of ice protection systems is on aircraft. During flight, the surfaces of an aircraft can be exposed to water vapour at low temperatures and, if no preventative action is taken, ice can quickly form on the wings, on control surfaces, and on other parts of the aircraft in such a way as to alter the aerodynamic performance of the aircraft (for example by altering the airflow around the aircraft and by adding additional weight to it) with potentially catastrophic consequences.
Electrothermal ice protection systems comprise a large number of heater devices (such as heater mats), which can be used as anti-icing zones in which a sufficient temperature is maintained at the surface of the wing in order to prevent the formation of ice on and behind the protected zone. These heater devices can also be used as de-icing zones to shed ice that has been allowed to accrete on the protected region. The de-icing mats are cyclically energised in order to melt the interface between the wing and the accreted ice, causing the ice to be shed.
In such an ice protection system it is important to avoid overheating of the heater devices (heating mats) in order to avoid a failure either of the devices or in the structure to which the devices are attached (this is known as an ‘overtemperature’ condition). Many modern aircraft (and other structures) use composite materials, which can suffer damage (delamination of the material, for example) at a relatively low temperature. Temperature ‘overshoot’ of the heater devices must therefore be controlled whilst maintaining rapid heating of the protected surface(s). At the same time, the temperature of the heater mat and external surfaces must not fall below the critical temperature at which ice shedding starts to occur (known as an ‘undertemperature’ condition).
Aircraft are normally subject to a range of different icing conditions during flight, such as different air temperatures, air velocities, relative humidity, and so on, which can depend for example on the location, altitude, orientation, air speed or pitch of the aircraft, the prevailing meteorological conditions, and so on. Different icing conditions can determine not only the temperatures and velocities (and so on) at which ice will form on different parts of the aircraft structure, but also the heat loss from the aircraft structure.
One approach to avoiding the temperature overshoot problem is disclosed in International Patent Application No. WO 2007/107732, the content of which is incorporated herein by reference. This document describes an ice protection system which uses a controller to maintain the temperature of a heater element (a heater mat) at a constant temperature regardless of the icing conditions such that, under a worst-case icing scenario, the power dissipated by the heater mat is sufficient to maintain the surface of the aircraft above a minimum temperature for de-icing. A temperature probe is embedded behind the heater element and records a temperature that is essentially identical to the temperature of the heater element (because the temperature gradient behind the heater element is much shallower than the temperature gradient between the heater element and the exposed surface). The controller operates by attempting to maintain the temperature at the temperature probe at a constant demand (or ‘setpoint’) temperature, which in turn maintains a substantially constant heater mat temperature.
One problem encountered with this system is that heater mats have some variability in thickness due to manufacturing tolerances. There can also be variation in the thickness of the dielectric layers surrounding the heater mat (and the like). This variability in thickness and a number of other factors can cause local ‘hot’ and ‘cold’ spots in the heater mat. If the temperature sensor is located behind a ‘hot’ spot or ‘cold’ spot, the temperature control can overall be a significant number of degrees centigrade too cold or too hot, respectively. This can lead to the undertemperature and overtemperature conditions mentioned above. If other heater mats are ‘slaved’ to the same controller, then the manufacturing tolerances ‘stack up’, and the permissible variation in temperature from the ideal can be relatively small.